Hi Lucy,
Im not sure why your resin would be coming out with marks on the side. You didnt mention if you were using a plastic or silicone mold. I would make sure to use a good mold release for either, which may help demolding future castings.
You also didnt mention what kind of resin you were using. Since youre getting scuff marks, I would suspect youre using epoxy. Epoxy resins are relatively soft and dont do well when buffed with a tools and compound. To get a shiny surface again, here are a few suggestions: recoating with another layer of resin, coating with a layer of our resin gloss sealer spray, buffing by hand with our Novus polishing kit. The first two options are more labor intensive, but will give you the shiniest results. Using the Novus polishing kit is almost as good, but can be done in a couple of minutes.
Good luck!